// Copyright (c) 2006-2009 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "utils/safe_strerror_posix.h"

#include <errno.h>
#include <stdio.h>
#include <string.h>

namespace dsn {
namespace utils {
#if defined(__GLIBC__) || defined(OS_NACL)
#define USE_HISTORICAL_STRERRO_R 1
#else
#define USE_HISTORICAL_STRERRO_R 0
#endif

#if USE_HISTORICAL_STRERRO_R && defined(__GNUC__)
// GCC will complain about the unused second wrap function unless we tell it
// that we meant for them to be potentially unused, which is exactly what this
// attribute is for.
#define POSSIBLY_UNUSED __attribute__((unused))
#else
#define POSSIBLY_UNUSED
#endif

#if USE_HISTORICAL_STRERRO_R
// glibc has two strerror_r functions: a historical GNU-specific one that
// returns type char *, and a POSIX.1-2001 compliant one available since 2.3.4
// that returns int. This wraps the GNU-specific one.
static void POSSIBLY_UNUSED wrap_posix_strerror_r(char *(*strerror_r_ptr)(int, char *, size_t),
                                                  int err,
                                                  char *buf,
                                                  size_t len)
{
    // GNU version.
    char *rc = (*strerror_r_ptr)(err, buf, len);
    if (rc != buf) {
        // glibc did not use buf and returned a static string instead. Copy it
        // into buf.
        buf[0] = '\0';
        strncat(buf, rc, len - 1);
    }
    // The GNU version never fails. Unknown errors get an "unknown error" message.
    // The result is always null terminated.
}
#endif // USE_HISTORICAL_STRERRO_R

// Wrapper for strerror_r functions that implement the POSIX interface. POSIX
// does not define the behaviour for some of the edge cases, so we wrap it to
// guarantee that they are handled. This is compiled on all POSIX platforms, but
// it will only be used on Linux if the POSIX strerror_r implementation is
// being used (see below).
static void POSSIBLY_UNUSED wrap_posix_strerror_r(int (*strerror_r_ptr)(int, char *, size_t),
                                                  int err,
                                                  char *buf,
                                                  size_t len)
{
    int old_errno = errno;
    // Have to cast since otherwise we get an error if this is the GNU version
    // (but in such a scenario this function is never called). Sadly we can't use
    // C++-style casts because the appropriate one is reinterpret_cast but it's
    // considered illegal to reinterpret_cast a type to itself, so we get an
    // error in the opposite case.
    int result = (*strerror_r_ptr)(err, buf, len);
    if (result == 0) {
        // POSIX is vague about whether the string will be terminated, although
        // it indirectly implies that typically ERANGE will be returned, instead
        // of truncating the string. We play it safe by always terminating the
        // string explicitly.
        buf[len - 1] = '\0';
    } else {
        // Error. POSIX is vague about whether the return value is itself a system
        // error code or something else. On Linux currently it is -1 and errno is
        // set. On BSD-derived systems it is a system error and errno is unchanged.
        // We try and detect which case it is so as to put as much useful info as
        // we can into our message.
        int strerror_error; // The error encountered in strerror
        int new_errno = errno;
        if (new_errno != old_errno) {
            // errno was changed, so probably the return value is just -1 or something
            // else that doesn't provide any info, and errno is the error.
            strerror_error = new_errno;
        } else {
            // Either the error from strerror_r was the same as the previous value, or
            // errno wasn't used. Assume the latter.
            strerror_error = result;
        }
        // snprintf truncates and always null-terminates.
        snprintf(buf, len, "Error %d while retrieving error %d", strerror_error, err);
    }
    errno = old_errno;
}

void safe_strerror_r(int err, char *buf, size_t len)
{
    if (buf == NULL || len <= 0) {
        return;
    }
    // If using glibc (i.e., Linux), the compiler will automatically select the
    // appropriate overloaded function based on the function type of strerror_r.
    // The other one will be elided from the translation unit since both are
    // static.
    wrap_posix_strerror_r(&strerror_r, err, buf, len);
}

std::string safe_strerror(int err)
{
    const int buffer_size = 256;
    char buf[buffer_size];
    safe_strerror_r(err, buf, sizeof(buf));
    return std::string(buf);
}
}
}
